The present invention relates to a method for operating a textile machine producing cross-wound bobbins and, more particularly, a spinning machine with a plurality of spinning stations each case having a take-off device with a take-off roller and a pressure roller for drawing off a yarn from a spinning device and a sensor mechanism for checking piecings.
German Patent Publication DE 42 35 450 A1 describes a method for determining the diameter of a bobbin at a spinning station of a spinning machine, in which method the speed of the feed roller, the rotor speed, the speed of the take-off and the winding roller are detected by means of sensors to determine the bobbin diameter. The yarn length which is used as a basis for a specific bobbin diameter is empirically determined by means of these variables. The empirically determined yarn length is used as a reference value here to which the actually measured yarn length is related. In this context, German Patent Publication DE 42 35 450 A1 names external influences, the occurrence of which influences the determination of the bobbin diameter. The use of pressure rollers with a different degree of hardness and which are possibly run-in is mentioned as an influencing factor of this type, inter alia. Pressure rollers worn in this manner result in the occurrence of different slip at the take-off rollers. The irregular wear of entraining rubbers on friction rollers is mentioned as a further influencing factor, which results in a different winding tension of the yarn. To compensate the influence of these factors on the bobbin diameter to be determined, German Patent Publication DE 42 35 450 A1 provides for the formation of correction values, which enter the determination of the diameter. The determination of the correction values described in German Patent Publication DE 42 35 450 A1 does not, however, supply any indication as to how wear of this type and its progress could be recognised.
Thus, in particular in the case of textile machines, the spinning stations of which carry out the piecing, process automatically without the aid of a so-called piecing or maintenance carriage, it is not only important to know about the current state of wear of pressure rollers against the background of determining the diameter of a bobbin, but also against the background of the quality of the piecings. Thus, for example, an inadequate parallelity of the pressure roller to the take-off element, caused by an irregular wear of the pressure roller, leads to an influencing of the piecing. This influencing can lead to the piecing not having the strength of the remaining spun yarn. An increasing take-off slip caused by the pressure roller wear may be expressed in the case of an open-end rotor spinning machine in that the piecing undergoes a higher increase in mass and a greater twist because of the higher residence time of the piecing in the spinning rotor and in the fiber flow. The higher mass causes the rotation to be deflected substantially into the thinnest point, the normal yarn region upstream of the piecing which leads to an over-twisting and therefore a weakening in this region. In order to be able to prevent this situation, knowledge about the state of wear of the pressure roller is of particular importance, in particular for the piecing process. The visual check of pressure rollers which is carried out represents, however, a disproportionate effort which is also unreliable with regard to its significance about the current state of wear.